1. Field of the Invention
This invention relates to improvements in a porous ceramic material and a method of producing the porous ceramic material, and a valve unit including a slidable member formed of the porous ceramic material, in which the porous ceramic material is high in durability under thermal shock and thermal stress while maintaining a high sliding characteristics even upon a long time use.
2. Description of the Prior Art
Hitherto a variety of methods have been proposed and put into practical use in order to improve a sliding characteristics of a slidable member, for example, of a valve of a combination faucet (for hot water and cool water) or of a mechanical seal ring. One of the methods is to form the slidable member of a material which is prepared by impregnating a porous ceramic material having a three-dimensional network structure, with lubricating oil. Another one is to select the smoothness of the sliding surface of a slidable member formed of a porous ceramic material, to fall below a predetermined value as disclosed in Japanese Patent Provisional Publication No. 6-58434. A further one is to select the surface roughness of the sliding surface of a slidable member formed of a porous ceramic material, to fall within a predetermined range as disclosed in Japanese Patent Provisional Publication No. 6-32646. According to the above methods, the sliding characteristics of the slidable member can be improved under the action of the lubricating oil to be exuded to a sliding surface, or under the action of pores located at and open to the sliding surface. In general, such a porous ceramic material is produced as follows: First, resin particles formed of a synthetic resin such as epoxy resin, phenolic resin or acrylic resin are mixed in ceramic powder to prepare a mixture. The mixture is then formed into a predetermined shape by using a metallic mold cress or the like thereby to form a formed body in which resin particles are dispersed. Finally, the formed body is sintered to form the porous ceramic material. During this sintering, the resin particles in the formed body is burnt or decomposed to be extinguished so that pores are formed in the ceramic material.
However, such a porous ceramic material has a tendency to be not always high in thermal shock resistance as compared with a ceramic material formed with no pores even though it is improved in sliding characteristics.